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The Phase Race Begins

At the risk of repeating myself, in summary, T2 relaxation comes from random causes while T2* comes from a combination of both random and fixed causes. There is nothing that can be done to prevent or compensate for random losses in phase, but what about the fixed effects? Can anything be done about these losses? The answer is yes.

Consider the following race. The contestants are a turtle, a bicyclist, you in the pace car, an airplane and a rocket. At the start of the race, everyone is together (in phase.) Once the race starts (at t = 0), the contestants all move out, each at their fastest pace. Soon, there is a noticeable distance between them.

The Phase Race Ends

(It's a Tie!)

After some time, let's call it TE/2, (Isn't that a great name?) a signal is given for everyone to turn around and go back. Assuming that everyone is still going at the same rate as before, then after an additional time, TE/2, they all arrive at the starting/finish line together.

In terms of MRI, at the time TE (TE/2 + TE/2) all the spins are back in phase, producing a large signal. This large signal is called a SPIN ECHO and the time TE is called the ECHO TIME.

Echo: The reflection (mirror image) of a signal caused by some sort of reversal of direction. (Think of sound bouncing off of a cliff.) In MRI, there are SPIN ECHOES and GRADIENT ECHOES. More on these to come



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